1. Field of the Invention
The present invention relates to vehicle control and, more specifically, to control of a vehicle with a brake hold control function by which braking force is held even when a brake pedal is not stepped on by the driver.
2. Description of the Background Art
Recently, a vehicle has been proposed that has automatic transmission and executes control of holding braking force at a halt (brake hold control) even if a brake pedal is not operated, when degree of brake pedal operation (for example, amount of operation of the brake pedal) exceeds a prescribed degree while the vehicle is stopped in the forward position, in order to alleviate load of braking operation on the user, for example, in congestion. In the vehicle in which the brake hold control is executed, even when the driver moves his/her foot away from the brake pedal, braking force of the stopped state is maintained. The brake hold control is released and the vehicle can be moved forward when the driver operates an accelerator pedal. Therefore, on a climbing road, the vehicle does not move backward even when the driver moves his/her foot off from the brake pedal and, hence, the starting is easier. On a downhill, the brake hold control is released when the accelerator pedal is stepped on and, therefore, acceleration due to the decline might be added to engine acceleration, possibly causing sudden starting. As a solution to this problem, a technique is disclosed in Japanese Patent Laying-Open No. 10-329671.
The brake control system disclosed in Japanese Patent Laying-Open No. 10-329671 has the brake hold function by which brake pressure applied from a master cylinder linked to the brake pedal is maintained even when one's foot is moved off from the brake pedal while the vehicle is stopped. The brake control system includes a gradient judging part for judging gradient of a road at least based on accelerator pedal position of the vehicle, and a control part for releasing the brake pressure holding state if the gradient of the road is downhill.
According to the brake control system disclosed in Japanese Patent Laying-Open No. 10-329671, the gradient of road is determined at least based on the accelerator pedal position and, if it is a downhill, the brake pressure holding state is released. Therefore, when the vehicle is started on a downhill slope, the brake pressure holding state has already been released. Accordingly, sudden starting of the vehicle because of simultaneous application of engine acceleration and acceleration due to the decline can be prevented.
In order to improve accuracy of detecting an acceleration request by the driver, sometimes it is determined that an acceleration request is made by the driver when the accelerator pedal position attains to a predetermine value, not when the accelerator pedal is simply operated, and on this determination, the brake hold control is released and driving force is output. In this situation, it follows that the driving force is output slightly delayed from the timing of operation of the accelerator pedal. Further, in order to avoid abrupt change in direction of acting torque applied to a drive line, for example, it is necessary to limit increase of driving force per unit time at the start of the vehicle. If such limit should be imposed when the brake hold control is released, the timing of output of the driving force in responses to the driver's request would further be delayed, and the vehicle starting characteristic as desired by the driver would not be realized.